In the last two decades, a variety of inorganic nanocrystals have been designed, synthesized and characterized, with the ultimate goals of developing a fundamental understanding of some of their unique chemical, physical and optical properties while exploiting the potential they offer in applications ranging from electronic devises to biology.1-7 Their properties often exhibit size and composition dependence, and are not shared by their bulk parent material or at the molecular scale.4-7 
These unique and controllable properties have permitted researchers across different fields to overcome some of the limitations encountered by conventional (bulk and such) materials for expanding old and developing new technologies. Among these nanostructured materials, pure and metal-doped iron oxide nanocrystals constitute one of the most exciting platforms due to their size-and composition-dependent magnetic properties. They have generated a great of deal of interest for use as magnetic resonance imaging (MRI) contrast agents, in magnetic guidance and/or separation, and as biological platforms for intracellular imaging.7-10 
In the early stage of development (late 1980s), large superparamagnetic iron oxide nanoparticles (SPIO, with dimensions >100 nm) containing several Fe3O4 nanocrystals were developed as in vivo T2 MRI contrast agents.9 More recently, and thanks to some remarkable improvements in the synthesis of high-quality nanocrystals using high temperature growth methods, preparation of several iron oxide-based nanocrystals, with demonstrated control over size-and composition-dependent magnetic properties have been reported.11-13 
This control has intensified interest in further enhancing the contrast efficiency and understanding the biological distribution of these materials inside organisms.13,14 However, issues of biological targeting, biodistribution and in vivo toxicity of nanomaterials, in general, greatly depend on their stability in complex biological media, their biocompatibility, and their hydrodynamic dimensions. These properties are directly controlled by one's ability to interface them effectively and reproducibly with biological systems.
Any nanoparticle platform with potential for use in biomedical applications should satisfy a few requirements, namely: 1) the surface coating of the nanoparticle should promote biocompatibility and reduce non-specific interactions while maintaining a compact size; 2) the nanoparticle should exhibit long-term stability in the presence of high electrolyte concentrations and over a broad pH range; and 3) the nanoparticles should have effective and controllable surface functionalization, which permits control over the number and nature of biomolecules attached to the nanoparticles, thus facilitating their use in applications such as targeting, sensing, and imaging.
The most effective synthetic strategies for obtaining high quality magnetic nanocrystals are based on a high temperature reaction of organometallic precursors. These strategies provide nanocrystals that are dispersible mainly in hydrophobic solutions, i.e., water-immiscible nanoparticles. Thus, additional processing using surface ligand exchange or encapsulation within phospholipid micelles or block copolymers is required to transfer these materials to buffer media and to impart biocompatibility. For instance, cap exchange with bifunctional hydrophilic ligands is simple to implement and can produce compact hydrophilic platforms.13-18 Nonetheless, these strategies often rely on the use of commercially available but ineffective ligands or large mass block copolymers. These approaches provide nanoparticles with limited long term stability and/or substantially increased hydrodynamic size.
It has been demonstrated that catechol derivatives such as the neurotransmitter dopamine and L-3,4-dihydroxyphenylalanine (L-DOPA), a precursor to dopamine that is also used as a component of adhesives generated by marine mussels, exhibit strong affinity to metal oxide nanocrystals.16,19,20 Several recent studies have reported that catechol-appended single chain PEGs provide effective capping ligands for iron oxide nanocrystals and permit their transfer to aqueous media. Catechol-PEG-capped iron oxide nanoparticles also have been used in cellular labeling and targeted MR imaging studies17,18,21-23.
Although these coating polymers are water dispersible, improvement is needed.